The drilling of wells, for example, for oil and gas production conventionally employs relatively small diameter strings of drill pipe to which is secured a drill bit of somewhat larger diameter. After a selected portion of the wellbore has been drilled, the well bore is usually lined with a string of tubulars known as casing. The casing (herein used to encompass any wellbore liner) has a smaller diameter than the drill bit. This conventional system which requires sequentially drilling the borehole using drill pipe with a drill bit attached thereto, pulling the drill pipe out of the borehole and running casing into the borehole is time consuming and costly. In addition, control of the well is difficult during the period that the drill pipe is being removed and the casing is being run in.
Drilling with casing is gaining popularity as a method for drilling wherein the casing is used as the drill string and drilling conduit and, after drilling, the casing remains downhole to act as the wellbore liner. A drilling assembly, including a drill bit and one or more hole enlargement tools such as, for example, an underreamer, is used which drills a borehole of sufficient diameter to accommodate the casing. The drilling assembly is deployed on the advancing end of the casing. The drill bit can be retractable and/or removable through the casing.
Casing drilling has been tested for drilling vertical, straight and deviated wellbores.
Recently, as described in copending PCT application s.n. PCT/CA99/00636, a process has been invented for drilling directional holes using casing as a drill string. According to that process a directional borehole assembly and a drilling assembly are connected to the distal end of a drill string. The directional borehole assembly includes a biasing means for applying a force to the drilling assembly to drive it laterally relative to the wellbore. The directional borehole and drilling assemblies can include drill bits with rotary steerable tools or downhole motors equipped with bent housings and/or bent subs that permit control of forces acting perpendicular to the drill string to steer the drill bit in a selected direction while drilling.
In liner drilling, the drilling assemblies operate and advance to extend the borehole while being mounted on the end of a section of liner. The liner is connected to surface by a length of drill pipe or additional casing.